US 7447187 B2
A technique for encoding digital communication signals. Data symbols are augmented in pilot symbols inserted at predetermined positions. The pilot augmented sequence is then fed to a deterministic error correction block encoder, such as a turbo product coder, to output a coded sequence. The symbols in the error correction encoded sequence are then rearranged to ensure that the output symbols derived from input pilot symbols are located at regular, predetermined positions. As a result, channel encoding schemes can more easily be used which benefits from power of two length block sizes.
1. A method for encoding an input digital bit stream for communication over a channel, the channel being defined by coding, the method comprising:
selecting a group of input digital bits as an input symbol block, and forming data symbols therein;
inserting pilot symbols in the input symbol block, the pilot symbol being inserted into predetermined positions within the input symbol block;
encoding the symbol block with a systematic block coder, the systematic block coder producing an encoded symbol block which includes data symbols, pilot symbols, and parity symbols in deterministic locations; and
interleaving the pilot symbols within the encoded symbol block to produce an output symbol block consisting of data symbols and parity symbols, as well as pilot symbols located at intervals within the output symbol block.
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modulating the output symbol block with coding to provide a channel signal.
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storing the data and parity symbols in the encoded symbol block in a temporary storage matrix in row order; and
reading out the contents of the temporary storage matrix in column order, and augmenting each such column with a pilot symbol.
9. A method for decoding a received symbol stream into an output digital bit stream, the symbol stream including at least one output symbol block, the output symbol block including data symbols, pilot symbols, and parity symbols, the method comprising:
selecting an output symbol block within the received symbol stream;
de-interleaving the pilot symbols located at intervals within the output symbol block to produce an encoded symbol block that includes data symbols, pilot symbols, and parity symbols in deterministic locations;
decoding the encoded symbol block using a systematic block decoder, the systematic block decoder producing a symbol block which includes data symbols and pilot symbols in deterministic locations; and
removing the pilot symbols in the symbol block, the pilot symbols being removed from predetermined positions within the symbol block to produce a group of output data symbols that forms a group of output digital bits of the output digital bit stream.
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generating a reference pilot signal and multiplying the reference pilot signal with the data in the pilot stream.
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demodulating a signal received from a communications channel defined by coding to produce the received symbol stream.
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17. An apparatus for encoding an input digital bit stream for communication over a channel, the channel being defined by coding, the input digital bit stream being grouped into input symbol blocks with data symbols therein, the apparatus comprising:
a pilot inserter unit that inserts pilot symbols in the input symbol block, the pilot symbol being inserted into predetermined positions within the input symbol block;
a systematic block encoder unit that produces an encoded symbol block which includes data symbols, pilot symbols, and parity symbols in deterministic locations; and
an pilot interleaver unit that interleaves the pilot symbols within the encoded symbol block to produce an output symbol block comprising data symbols and parity symbols, as well as pilot symbols located at intervals within the output symbol block.
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25. An apparatus for decoding a received symbol stream into an output digital bit stream, the symbol stream including at least one output symbol block, the at least one output symbol block being selected for decoding, the output symbol block including data symbols, pilot symbols, and parity symbols, the apparatus comprising:
a pilot deinterleaver unit that de-interleaves the pilot symbols located at intervals within the output symbol block to produce an encoded symbol block that includes data symbols, pilot symbols, and parity symbols in deterministic locations;
a systematic block decoder unit that decodes the encoded symbol block, the systematic block decoder producing a symbol block which includes data symbols and pilot symbols in deterministic locations; and
a pilot removal unit that removes the pilot symbols from the symbol block, the pilot symbols being removed from predetermined positions within the symbol block to produce a group of output data symbols that forms a group of output digital bits of the output digital bit stream.
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This application is a continuation of U.S. application Ser. No. 09/728,575, filed Nov. 30, 2000, now U.S. Pat. No. 6,804,223 the entire teachings of which are incorporated herein by reference.
The present invention relates to communications systems and in particular to a scheme for digital encoding of signals in a wireless system.
Demand for wireless communications equipment and services continues to grow at an unprecedented rates throughout the world. Increasingly, such systems are commonly relied upon to provide voice and data communications to a growing sector of the public. While these systems originally depended upon analog signaling technology, there is essentially unanimous agreement that future systems will be based on various types of digital signal coding schemes.
The typical wireless communication system is a point to multi-point type system in which a central base station communicates with a number of remote units located within a local geographic area of coverage known as a cell. This system provides for duplex communication such that signals may be sent in both a forward direction (from the base station to the remote unit) as well as in a reverse direction (from the mobile remote unit back to the base station). In order to support communication between the remote unit and networks such as the Public Switched Telephone Network (PSTN), or data networks such as the Internet, the wireless system must also provide for various logical components and functional entities.
Consider the Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) digital systems presently in widespread use. Each of these systems provide for certain logical types of the radio channels that make up the forward link and reverse link. In particular, the forward link channels often include a pilot channel, paging channels, and multiple forward traffic channels. The traffic channels are used to carry the payload data between the base station and the mobile unit. A pilot channel is also typically required to allow the remote unit to maintain synchronization with the base station. The paging channels provide a mechanism for the base station to inform the remote unit of control information, such as the assignment of forward traffic channels to particular connections and/or subscriber units.
Likewise, an access channel is provided in the reverse direction in addition to reverse traffic channels. The access channels allow the remote units to communicate control information with the base station, such as to send messages indicating the need to allocate or deallocate connections as required.
Various environmental conditions will affect the performance of any wireless communications system. These elements include atmospheric signal path loss, which may often introduce fading and interference. Fading may include variations that are introduced as a result of the specific terrain within the cell, as well as other types of fading, such as multi-path fading, that occurs due to signal reflections from specific features, such as buildings that cause fluctuations in receive signal strength. Systems in which the remote unit may be a mobile unit, especially those potentially operating at higher speeds, such as the cellular telephones used in automobiles, are particularly susceptible to multi-path fading. In such an environment, the signal pathways are continually changing at a rapid rate.
Certain techniques can be used to attempt to eliminate the detrimental effects of signal fading. One common scheme is to employ special modulation and/or coding techniques to improve the performance in a fading environment. Coding schemes such as block or convolutional coding add additional parity bits at the transmitter. These coding schemes thus provide increased performance in noisy and/or fading environments at the expense of requiring greater bandwidth to send a given amount of information.
In addition, pilot signals may also be used to provide a reference for use in signal demodulation. For example, most digital wireless communications systems provide for a dedicated pilot channel on the forward link. This permits the remote units to remain in time synchronization with the base station. Certain systems, such as the IS-95 CDMA system specification promulgated by the Telecommunications Industry Association (TIA) uses the pilot signals that include pseudorandom binary sequences. The pilot signals from each base station in such a system typically use the identical pseudorandom binary sequence, with a unique time offset being assigned to each base station. The offsets provide the ability for the remote stations to identify a particular base station by determining this phase offset in the forward link pilot channel. This in turn permits the remote units to synchronize with their nearest neighboring base station. Coding the pilot channel in this way also helps support other features, such as soft handoff for cell-to-cell mobility.
The pilot signal, having a predictable frequency and rate, allows the remote units to determine the radio channel transfer characteristics. By making such determinations, the receiver may in turn further compensate for the distortion introduced in the channel during the process of estimating symbols being received.
However, it is generally considered to be impractical to use pilot signals in the reverse link. In particular, this would lead to a situation where pilot signal channels would have to be dedicated for each remote unit. While this would not necessarily pose a problem in a point to point system, in point to multi-point systems such as a cellular telephone network, the architecture would quickly lead to inefficiency in use of the available radio spectrum. In addition, it is generally thought that the overhead associated with a system that assigned individual pilot channels to each remote unit would unnecessarily complicate the base station receiver processing.
An alternative to allocating individual pilot channels is to make use of a sequence of pilot symbols. The pilot symbols are interleaved with data symbols on the traffic channel. This technique is generally referred to as pilot symbol assisted modulation. In such a system, the transmitter encodes the data to be sent on the traffic channel as a series of symbols. A pilot symbol interleaver then inserts a sequence of predetermined pilot symbols within the data symbol sequence. The pilot symbol augmented sequence is then modulated and transmitted over the radio channel. At the receiving station, a decimater or deinterleaver and filter separate the pilot symbols from the data symbols.
What is needed is a way to integrate pilot symbol assisted modulation techniques with block encoding schemes in a way in which maximizes the probability that data and pilot symbols will be correctly received.
The invention accomplishes this with a pilot symbol insertion scheme that proceeds as follows. The source data bits are first augmented with periodically inserted pilot symbols. In a preferred embodiment, the pilot symbols are inserted at a position corresponding to a power of two, such as for example, every fourth, eighth, sixteenth, or thirty-second symbol. Next, this pilot symbol augmented data sequence is presented to a deterministic block coder. Such a block coder may, for example, be a sub-rate two dimensional turbo product coder.
The symbols of the resulting encoded block are then rearranged such that the pilot symbols will be in a predictable location. Because the pilot symbols are always in a known place in the input block coding matrix, their positions are therefore also known in the output block coding matrix. The encoded output pilot symbols can therefore be rearranged such that they are evenly distributed through the output coded space, prior to modulation and transmission.
An optional embodiment makes use of an interleaving scheme in which parity symbols are interleaved with data and pilot symbols. In such a scheme, all symbols from the coded space, with the exception of the pilot symbols, are placed in a temporary storage area by row. Data is then read out of the temporary storage area to provide the interleaved output, by reading data from the temporary array in column order. For example, a first pilot signal is selected, a row is read out, a second pilot signal is selected, a second row is read out, and so on. As a result, the pilot symbols are output at predetermined positions preferably located within symbol positions which are a power of two away from each other.
In an alternate embodiment, the symbols may be composed of pairs of input data bits, to form complex-valved symbols, which can then be modulated using Quadrature Phase Shift Keyed (QPSK) schemes. In this embodiment, the data, parity, and pilot bits are processed in pairs.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.
The communication system 10 includes a number of Personal Computer (PC) devices 12-1, 12-2, . . . 12-h, . . . 12-l, corresponding Subscriber Access Units (SAUs) 14-1, 14-2, . . . 14-h, . . . 14-l, and associated antennas 16-1, 16-2, . . . 16-h, . . . 16-l. Centrally located equipment includes a base station antenna 18, and a Base Station Processor (BSP) 20. The BSP 20 provides connections to an from an Internet gateway 22, which in turn provides access to a data network such as the Internet 24, and network file server 30 connected to the network 22. The system 10 is a demand access, point to multi-point wireless communication system such that the PCs 12 may transmit data to and receive data from network server 30 through bi-directional wireless connections implemented over forward links 40 and reverse links 50. It should be understood that in a point to multi-point multiple access wireless communication system 10 as shown, a given base station processor 20 typically supports communication with a number of different subscriber access units 14 in a manner which is similar to a cellular telephone communication network.
The PCs 12 may typically be laptop computers 12-l, handheld units 12-h, Internet-enabled cellular telephones or Personal Digital Assistant (PDA)-type computers. The PCs 12 are each connected to a respective SAU 14 through a suitable wired connection such as an Ethernet-type connection.
An SAU 14 permits its associated PC 12 to be connected to the network file server 30 through the BSP 20, gateway 22 and network 24. In the reverse link direction, that is, for data traffic traveling from the PC 12 towards the server 30, the PC 12 provides an Internet Protocol (IP) level packet to the SAU 14. The SAU 14 then encapsulates the wired framing (i.e., Ethernet framing) with appropriate wireless connection framing. The appropriately formatted wireless data packet then travels over one of the radio channels that comprise the reverse link 50 through the antennas 16 and 18. At the central base station location, the BSP 20 then extracts the radio link framing, reformatting the packet in IP form and forwards it through the Internet gateway 22. The packet is then routed through any number and/or any type of TCP/IP networks, such as the Internet 24, to its ultimate destination, such as the network file server 30.
Data may also be transmitted from the network file server 30 to the PCs 12 in a forward direction. In this instance, an Internet Protocol (IP) packet originating at the file server 30 travels through the Internet 24 through the Internet gateway 22 arriving at the BSP 20. Appropriate wireless protocol framing is then added to the IP packet. The packet then travels through the antenna 18 and 16 to the intended receiver SAU 14. The receiving SAU 14 decodes the wireless packet formatting, and forwards the packet to the intended PC 12 which performs the IP layer processing.
A given PC 12 and the file server 30 can therefore be viewed as the end points of a duplex connection at the IP level. Once a connection is established, a user at the PC 12 may therefore transmit data to and receive data from the file server 30.
The reverse link 50 actually consists of a number of different types of logical and/or physical radio channels including an access channel 51, multiple traffic channels 52-1, . . . 52-t, and a maintenance channel 53. The reverse link access channel 51 is used by the SAUs 40 to send messages to the BSP 20 to request that traffic channels be granted to them. The assigned traffic channels 52 then carry payload data from the SAU 14 to the BSP 20. It should be understood that a given IP layer connection may actually have more than one traffic channel 52 assigned to it. In addition, a maintenance channel 53 may carry information such as synchronization and power control messages to further support transmission of information over the reverse link 50.
Similarly, the forward link 40 typically includes a paging channel 41. The paging channel 41 is used by the BSP 20 to not only inform the SAU 14 that forward link traffic channels 52 have been allocated to it, but also to inform the SAU 14 of allocated traffic channels 52 in the reverse link direction. Traffic channels 42-1 . . . 42-t on the forward link 40 are then used to carry payload information from the BSP 20 to the SAUs 14. Additionally, maintenance channels carry synchronization and power control information on the forward link 40 from the base station processor 20 to the SAUs 14.
In the preferred embodiment, the logical channels 41-43 and 51-53 are defined by assigning each channel a unique pseudorandom channel (PN) code. The system 10 is therefore a so-called Code Division Multiple Access (CDMA) system in which channels assigned to unique codes may use the same radio carrier frequency. The channel may also be further divided or assigned. Additional information as to one possible way to implement the various channels 41, 42, 43, 51, 52, and 53 is provided in Patent Cooperation Treaty Application No. WO99/63682 entitled “Fast Acquisition Of Traffic Channels For A Highly Variable Data Rate,” assigned to Tantivy Communications, Inc., and published Dec. 9, 1999. In a preferred embodiment, the channel codes are a type of PN code which repeats at a code length of 2N. One such orthogonal PN code scheme is described in U.S. patent application Ser. No. 09/255,156, filed Feb. 23, 1999, entitled “Method and Apparatus for Creating Non-Interfering Signals Using Non-Orthogonal Techniques”, assigned to Tantivy Communications, Inc.
Turning attention now to
In any event, a transmitter 100 is implemented with a pilot inserter 110, block encoder 120, pilot interleaver 130, channel coder 140 and radio frequency (RF) modulator 150. The receiver 200 includes an RF demodulator 250, channel decoder 240, pilot deinterleaver 230, block decoder 220, pilot removal 210, and pilot reference generator 205.
It should be understood that the receiver 200 performs the inverse functions of the corresponding portions of the transmitter 100. In such an instance, the RF demodulator 250 performs the inverse radio frequency to modulation process, the channel decoder 240 decodes the channel codes reversing the operation of the channel coder 140, the pilot deinterleaver 230 performs the inverse function of the specific pilot interleaver 130 implemented in the transmitter, and the block decode process 220 also undoes the block encode process 120. The pilot removal process 210 uses a pilot reference signal generator 205, for example, to multiply the received data in pilot stream via reference pilot signal to further aid in the recovery of the data. A pilot inserter 110 typically makes sense in the reverse link 50 given and that pilot symbols are preferably inserted with the data symbols or bits in this same channel. This is opposed to an arrangement where there are separate pilot channels devoted separately for simply sending pilot signals, which is typically more practical on the forward link 40, in which case a single pilot channel can be associated with and be shared by numerous SAUs 14.
Before discussing the details of the pilot inserter 110 and block encoder 120 in more detail, it is instructed to consider the operation of a typical error coding process. In particular, consider an example situation in the use of a turbo product code which is to encode data at the rate of ¼. (We assume in the discussion of this first embodiment that data is real-valued only such that a “symbol” is a single data bit, and discuss a situation with complex-valued data symbols later on.) In the case shown in
For a typical prior art block coding operation, that is one without supplementation with pilot symbols according to the invention, the 16 input data bits are placed an upper left hand corner of the 8×8 encoded space as shown in
The encoded matrix is then presented to the block encoder to calculate and create the parity bits for an encoded space. In the example being discussed, in the case of a ¼ rate code, three times as many parity bits as data bits are calculated and created as shown in
Returning attention to
Turning attention to
As in the case of the standard turbo product code, the matrix in
It is the job of the pilot interleaver 130 to rearrange the output matrix in such a manner that the pilot symbols are evenly distributed among the data and parity symbols in a manner which makes sense. In the simplest instance, the data and parity symbols can be placed on the channel in, more or less, the order in which they appear in the matrix. This situation is shown in
In certain other instances it is important to interleave the parity symbols among the pilot and data symbols as well. In this situation, the parity symbols and data symbols can be better distributed throughout the information space. In this approach, all except the pilot symbols may be placed in a temporary storage area 180 in row order fashion and then read out by column order. The goal in allocating rows and columns in the temporary storage area 180 is to remain as square as possible. Thus, in the example illustrated shown in
In yet another example of the implementation of the interleaver 130, it may be advantageous to apply data to the channel with Quadrature Phase Shift Keyed (QPSK) format modulation. In this case, individual input data bits are read in pairs so that for example, 2 pilot bits are required to make up respectively the In-phase (I) and the Quadrature (Q) portion of a complex valued data symbol. In this case, pilot bits are also read out in pairs so that two pilot bits comprise a pilot symbol. The result, as shown in
Using the systematic block encoder, the position of the pilot, information, and parity symbols is always known in the output matrix. This creates a structure where pilot symbols can be repositioned in a known fashion, to ensure that they repeat in a regular pattern in the modulated output signal.
For example, a system timing requirement may demand that the ratio of pilot symbols to the ratio of data and parity symbols remain at a power of two, so that clock phasing requirements are much easier to meet. In particular, even if a block encoder produces a number of data and parity symbols as a power of 2, the additional pilot symbol insertions would create an output sequence which is not an exact power of 2. This makes it difficult to insert pilot symbols in blocks which do not remain in phase, and therefore “roll” with respect to the PN sequences used with respective channel encoding 140. For example, in a case where pilot symbols need to be inserted 6.25% of the time, a block of 4096 would require 4096 for data and parity, plus 6% of 4096, or 256 symbols for pilots for a total of 4352 symbols per block. Because no PN channel code is such a length, to maintain synchronization, the position of the PN code would change at the start of every symbol block. However, with the invention, this difficulty is avoided, and the output symbol blocks are easily contrived to be in groups of 2N, including both parity and pilot symbols. Thus, PN code synchronization timing, as required to maintain the proper spread spectrum characteristics, is easy.